Certain "load-moving" drive systems involve two or more electric motors for moving a single load. In some systems, the motors exert forces on the load by some sort of "resilient" connections, e.g., elastic belts or slip clutches. In such a configuration, each motor (and the power transmission and control equipment connected thereto) is somewhat insulated from mechanical shock resulting from a failure of the motors to share load equally.
On the other hand, some types of systems couple plural motors to a load using "rigid" connections such as gear boxes, line shafts or the like. In those instances, failure of motors to substantially equally share the load can result in undue stress to all of the mechanical components in the drive train (including the motors themselves) and to the electrical control system. Premature wear and breakage is often the result.
An example of an application involving plural drive motors for moving a single load is a crane. Cranes are made in a wide variety of configurations. One type is called an overhead travelling crane (OTC) and is used in factories or the like for lifting and placing loads on the factory floor. Such a crane travels on a pair of elevated main rails which are parallel and spaced apart, usually by several yards. A pair of bridge girders spans across the rails and there are driven wheels mounted at either end of the girders for riding atop the rails. And the girders themselves have rails on them.
A "trolley" is mounted on the girder rails and can travel the width of the bridge under motive power. A load hoist is mounted on the trolley and includes a powered hoist/lower "rope drum" about which steel cable is wrapped. The cable is connected to a load-lifting hook, sling, magnet or the like. With the foregoing arrangement, the operator (who usually rides in a cab attached to the crane) can pick up, move and deposit a load anywhere in the area travelled by the crane.
Another type of crane is called a straddle crane and many of its operating principles are similar to those of an OTC. A difference is that the main rails are about at ground level and the trolley is supported at an elevation by legs extending down from the bridge girders to the main rails. A straddle crane resembles an inverted letter "U" in shape. An advantage of such an arrangement is that the main rails need not be mounted at an elevated level. Another is that the crane can position itself over (or "straddle") a truck, for example, and unload the truck's cargo. Sometimes the crane is an L-shaped "hybrid" with one end supported on an elevated rail and the other on a rail at ground level.
In load-moving systems of the foregoing types (including load hoisting/lowering cranes), controllability of load "picking" and placement is important. That is, the system operator must have the ability to gently initiate load movement and later place the load accurately and gently. Nowhere is this more true than in crane material handling work where the load to be lifted and placed can be an expensive machined product worth thousands of dollars. Such cranes are sometimes designed to lift tons, even hundreds of tons, of load. And, often, crane operations are attended by workers at ground level who assist in load attachment, placement and detachment. Such workers are understandably intolerant of a hoist/lower system which is difficult to control accurately. In plural-motor systems, good control and maximum load lifting capability depend in part upon equalized motor load sharing.
In former years, cranes of the foregoing type were often powered by direct current (DC) electric motors. More recently, alternating current (AC) drive systems have come into wide usage due in significant part to the fact that they can derive power from the existing AC power distribution networks.
A type of variable speed AC drive system used on cranes includes squirrel cage AC motors (generally acknowledged to be the most straightforward of all AC motor types) and controllers for providing variable motor speed ranging from "creep" to rated speed. While such drives are often known as adjustable frequency drives, in fact the power applied by the control system to the AC motor(s) is adjustable both in frequency and in voltage. The frequency of the applied voltage (which ranges generally from zero to 60 Hz for a 60 Hz network as in the United States) controls motor speed while the magnitude of the applied voltage controls motor current and therefore motor output torque. A known way of coordinating frequency and voltage is generally to cause a predetermined change in voltage with each, say, one Hz change in frequency. At very low frequencies, the voltage is elevated slightly to compensate for motor current/resistive losses, often called "IR losses." Some years ago, the term "Hertz," abbreviated "Hz," replaced the phrase "cycles per second" and is synonymous therewith.
A known AC adjustable frequency drive system, useful on cranes, includes a controller for two motors mechanically coupled to the same hoist drum by gearboxes. Such controller has a pair of general purpose, microprocessor-based adjustable frequency inverters (sometimes called scalar inverters), one for each motor. Each inverter derives input power from the AC network at, e.g., 460 volts, 3 phase, 60 Hz. Output power from the inverter to the connected motor is adjustable in frequency and voltage as described above.
A single master switch "tells" the system what motor speed is desired by the operator controlling such switch. It does so by providing a variable AC reference voltage nominally in the range of 0-16v. The master switch handle is movable in an arc either side of a center "neutral" position and the degree of handle displacement from the neutral position determines the magnitude of such voltage.
This voltage is directed to an interface card which rectifies it and responsively provides a DC motor speed signal nominally in the range of 0-10v. As the microprocessors of each inverter scan their programmed instructions, they periodically "read" this motor speed signal and responsively cause the inverters to apply electrical power to the motors at a frequency (and voltage) which correlates therewith. So-called "command ramps" are used to control the inverters in that way.
A command ramp is provided by the microprocessors according to aspects of the programming embedded therein. Such ramp, in the nature of an electrical signal, derives its name from the fact that it causes applied motor voltage to change somewhat gradually rather than instantaneously. One occasion in which such ramps affect system operation is when the operator rapidly throws the master switch handle "hard over" from neutral to an extreme position. Even though such position signifies full motor speed, the system prevents instantaneous application of full voltage. Rather, the applied motor voltage is "ramped up" to the value represented by the final master switch position. Such a system is sometimes said to provide "soft" acceleration and abuse to the electrical and mechanical components is thereby substantially reduced. A motor control system of the foregoing type is made by Harnischfeger Corporation of Milwaukee, Wisconsin, and sold under its SMARTORQUE.RTM. trademark.
With the foregoing arrangement, it is anticipated that the same voltage at the same frequency would be applied to both motors. The output torque of each motor is a function of motor electrical current, in turn a function of applied frequency and voltage. Therefore, electrical current is an indication of the degree to which the motors would share load. With equal motor currents, load sharing would be substantially equal.
Notwithstanding that both inverters are detecting the same motor speed signal, it has been observed that motor currents were not always equal to one another and the proportions of the total load being handled by each motor were sometimes disparate and varied randomly by as much as 1.5:1 or more. As a result, the crane is not able to easily lift rated load, one motor is somewhat "dragged along" by the other and both the electrical and mechanical systems were abused.
One solution to such a problem is to use special purpose inverters, the cost of which is significantly greater than that of a general purpose inverter. Another solution is to provide a feedback circuit whereby motor currents (or speeds or other parameters) are "fed back" for comparison with, say, the DC motor speed signal. Since such speed signal represents the desired motor operating condition, comparing it with a signal representing an actual motor operating condition yields (or may yield) an "error" signal. A feedback system is arranged to automatically change the actual operating condition until it satisfactorily matches that desired.
Such feedback systems are significantly more complex than similar non-feedback systems. As such, they are more expensive in the first instance, are more difficult to troubleshoot and somewhat more prone to failure.
A system which is inexpensive, easy to install or retrofit to existing controllers, uses general purpose inverters and is highly effective in causing load sharing in AC adjustable frequency drives would be an important advance in the art.